The present invention relates to a power supply circuit for an electronic apparatus employing a microcomputer.
Recently, various domestic appliances, such as a microwave oven, have been equipped with a microcomputer control system. The microcomputer controlled microwave oven is very effective because various cooking sequencies are automatically controlled.
The microcomputer control system comprises a central processor unit, a read only memory, and a random access memory. These elements are integrated in a single chip LSI. For the proper operation of such a microcomputer control system, all counters must be automatically cleared when the main power supply switch is thrown.
The microwave oven is supplied with power from the commercial power supply. Generally, the commercial power supply develops the sinusoidal alternating voltage of 50 Hz or 60 Hz. The microcomputer control system requires a D.C. voltage to perform its function and, therefore, a power supply circuit is employed to obtain a desired D.C. voltage. The power supply circuit includes a large capacity capacitor for regulation purposes and for protecting the microcomputer control system from a sudden interruption of the power supply. The large capacity capacitor slows down the response characteristics of the power supply and, therefore, the conventional power supply circuit exhibits a leading transient period longer than several tens of milliseconds.
On the other hand, the LSI microcomputer control system is normally designed to respond to the leading transient period shorter than 10 msec because of its considerably short time constant characteristics. The conventional switching regulator system is suited for shortening the leading transient period, but the switching regulator system requires a great number of circuit elements and inevitably generates noises because of high-frequency operation.
Accordingly, an object of the present invention is to provide a power supply circuit suited for an LSI microcomputer system.
Another object of the present invention is to provide an AC-DC converter showing a short leading transient period.
Still another object of the present invention is to provide a power supply circuit of a simple construction suited for developing an auto-clear signal in an LSI microcomputer system.
Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
To achieve the above objects, pursuant to an embodiment of the present invention, a rectifying circuit is provided for converting the alternating commercial power supply voltage to a D.C. voltage of a desired level. A switching circuit is interposed between the rectifying circuit and a single chip LSI microcomputer control system. A time constant circuit is further provided for switching on the above-mentioned switching circuit when an output voltage of the rectifying circuit reaches a preselected level. Thus, the power supply circuit supplies a voltage to the single chip LSI microcomputer control system with a short leading transient period.